1. Field of the Invention
The present invention relates to test coupons for determining creep characteristics for metal matrix composite structures, and more particularly to methods for forming and using such coupons and to configurations of the coupon itself.
2. Background of the Invention
The use of creep test coupons to determine structural properties and behavioral characteristics is not new. For many years, such coupons have been provided as samples of monolithic materials requiring investigation. Testing of the samples has been accomplished by subjecting them to compressive and/or tensile forces in appropriate testing apparatus, thereby making it possible to obtain data indicative of the character of materials in question.
Recently, new composite materials known as "metal matrix composites" (MMC's) have been developed for use in the aerospace industry, and there has arisen a great and urgent need for investigation of the properties of these materials as well.
In order to accomplish this task, the industry typically has performed creep and other types of material characteristic testing using a coupon of substantially rectangular shape having simple straight, parallel sides (see FIG. 1) or a coupon formed in a dog-bone shape (see FIG. 2).
To date, the latter configuration has been widely accepted as the industry standard. FIG. 3 illustrates this configuration of coupon when used with fiber-reinforced metal matrix materials. As is well known, the reinforcing fibers are embedded in the metal matrix material. The plurality of fibers include one or more layers of fiber sets, where in each layer the fibers are disposed at a predetermined angle (between 0.degree. and 90.degree.) relative to the longitudinal axis Z--Z of the coupon. Each set of angularly oriented fibers typically extends along a portion (if not all) of the length of the coupon. Those sets of fibers having no angular orientation relative to the coupon longitudinal axis (i.e., zero (0) degrees orientation) typically span the length of the coupon from one end region to the opposite end region.
Exemplary of such an angular arrangement of reinforcing fibers in a test coupon are the fiber sets 310, 320 and 330 shown in FIG. 3. Fiber set 310 is oriented at +45.degree. relative to the longitudinal axis Z--Z of the coupon, fiber set 330 is oriented at -45.degree. relative to the longitudinal axis, and fiber set 320 is positioned at an angular orientation of approximately zero degrees relative to the longitudinal axis Z--Z. The FIG. 3 representation of fiber sets does not purport to identify upper, middle and lower sets of fibers, but rather only is illustrative of three possible angular orientations of the fiber sets. Holes 302, 304 located in opposite end regions of the coupon serve as alignment means for facilitating attachment of the coupon in the testing fixture. As stated above, the three fiber sets depicted in FIG. 3 are merely intended to serve as illustrative examples of the infinite number of angular orientations possible. The number of fiber sets in any given test coupon, the different angular orientations of these fiber sets, and the relative placement of these fiber sets one above the other are all considered to be design considerations which depend on which, and to what degree of magnitude, strength-of-material characteristics are being developed.
Testing of coupons is typically accomplished by first securing opposite ends of the coupons in fixture clamps, and then subjecting the coupon to tensile or compressive forces, as the case warrants, to obtain the creep (or stress or strain) characteristics of the material involved.
One of the most disturbing problems attendant the use of the known fiber-reinforced MMC coupon configurations is that, after the coupon is clamped and the testing takes place, the test results produce creep curves which suggest that the section of fiber-reinforced metal matrix compound material located between the clamped end regions (i.e., in the gage section) is just as strong as the fibers disposed within the metal matrix compound material. In other words, an accurate reading of the test results with a high level of confidence cannot be obtained in carrying out standard testing procedures with the presently known test coupon configurations.
A situation in which the fiber-reinforced metal matrix compound material of the gage section would be expected to be as strong as the fibers in the MMC material would be where the coupon was perfectly fastened at both end regions and subjected only to a pure tension load. However, it would be presumptuous to believe that perfect loading or perfect clamping of the coupon could be attainable in practice. Indeed, it is believed that, in using the conventional coupon configurations and currently known procedures of clamping and testing coupons fabricated from fiber-reinforced metal matrix compound materials, creep strain (known to be recoverable in a monolithic material) would be trapped.